New energy vehicles are widely promoted in the country and even in the world. However, in order to completely replace fuel vehicles, there are many areas for improvement. For example, there are a few problems such as low travelling mileages of vehicles, high cost of battery packs, reliability of battery packs and the like, which need to be further solved.
Currently, a power battery usually adopts a square hard shell structure. A shell of the power battery includes a case and a cap assembly. The shell of the power battery provides a closed space for accommodating an electrode assembly and electrolyte. The power of the electrode assembly is led out of the closed space through a terminal of the cap assembly.
In a conventional cap assembly, a manner of fixing the terminal is as follows: a cap plate is provided with a through hole; the terminal includes a base portion and an extension portion; and the base portion has a cross-sectional area larger than an area of the through hole. During assembly, the base portion is located under the cap plate (i.e. inside the case), and the extension portion is fixed by a clamp spring or a riveting member after passing through the through hole. In this way, the terminal is fixed to the cap plate. With such a fixing manner, the cap assembly may employ a large number of mechanical components, thereby increasing the cost of the secondary battery and reducing the reliability of the cap assembly. Meanwhile, since the base portion is located inside the case, space utilization inside the case may be reduced and thus energy density of the power battery may be reduced. In order to solve this problem, a terminal board may be employed to be disposed at a side of the cap plate; a fixing member and a connecting member may be provided; the fixing member may be fixed to the cap plate through the connecting member; and at least part of an outer peripheral surface of the terminal board may be surrounded by the fixing member so that the terminal board may be fixed to the fixing member. However, when the cap plate is deformed to form an arch due to a large amount of gas generated inside the case of the secondary battery (the cap plate is deformed to form an arch because two edges of the cap plate in its width direction are welded to the case, the deformations of the two edges in the width direction are small, but the deformation of a middle region of the cap plate in the width direction is large), due to a large rigidity of the fixing member, the fixing member may not form an arch along with the cap plate, so the gaps between two sides of the cap plate in the width direction and the fixing member may be increased (even the connecting member may be pulled off). Thus it may not be possible to tightly press a sealing member between the terminal board and the cap plate, thereby causing air leakage or liquid leakage of the secondary battery.